The present invention relates to a method of manufacturing a preform for a nonoxide glass fiber and, more particularly, to a method of manufacturing a preform suitable for fabricating a nonoxide fiber such as halide glass material, chalcogenide glass material or the likk which are used in a long-distance optical-communication glass fiber, an infrared temperature measuring fiber, a laser-energy transmitting fiber or the like.
A nonoxide glass fiber is wide in a light transmitting window as compared with an optical fiber which is composed of an oxide glass material including a quartz glass material. Since, particularly, a light within an infrared range is well transmitted through the nonoxide glass fiber, it the has been expected that the nonoxide glass fiber is applied to various fields such as optical communication, temperature measurement, energy transmission, or the like. However, the nonoxide glass material is strong in crystallization tendency as compared with the oxide glass material, and the glass surface of the nonoxide glass material is active with respect to atmospheric oxygen and water. Further, since the nonoxide glass material is composed of multi-components which are largely different in vapor pressure from each other, attention must be paid to manufacturing and fiberization of a preform for an optical fiber having a core-cladding structure.
A built-in casting method, a rotational casting method and a double crucible method are known as a method of manufacturing the preform for the optical fiber made of the nonoxide glass material.
In the case of the built-in casting method, a molten cladding glass liquid is first poured into a casting mold so as to become full. By doing so, the molten glass liquid initiates solidification from its periphery. Before the solidification reaches the entirety, the casting mold is inverted or turned to put out or discharge the glass liquid at the central section of the casting mold. Subsequently, a core glass liquid is poured into the central section of the casting mold, and the entire core glass liquid is solidfied. Thus, there is produced the preform. In this connection, reference should be made to Japanese Patent Provisional Publication No. SHO 63-143508.
In the case of the rotational casting method, a small quantity of a cladding glass liquid is poured into a casting mold. The casting mold is rotated. A centrifugal force induced by the rotation causes the glass liquid to adhere to the inner wall surface of the casting mold. In this manner, the glass liquid is solidified, and a solidified glass material is formed along the inner wall surface of the casting mold. Subsequently, a core glass liquid is fully poured into the solidified glass material, and is solidified. Thus, there is produced a preform. In this connection, reference should be made to Japanese Patent Publication No. SHO 61-21174.
In the case of the double crucible method, so-called double crucibles are used in which a core glass liquid is poured into an internal crucible, and a cladding glass liquid is poured into an external crucible. These molten liquids pass continuously through their respective nozzles. Thus, there is produced a preform. In this connection, reference should be made to Japanese Patent Provisional Publication No. SHO 63-190741.
However, the above-described methods have the following various problems. That is, since both the cladding glass material and the core glass material are cast into the mold under the condition that the glass materials are low in viscosity, that is, are molten, bubbles and striae tend to be mixed into the glass materials. Furthermore, since the core glass liquid is further cast onto the cladding glass material under the condition of beginning to be solidified, the cladding glass material is again heated. Thus, since the cladding glass material is lengthened in remaining time within a range of crystal deposition or precipitation temperature, the cladding glass material tends to be crystallized. Moreover, a halide glass material and a chalcogenide glass material have the following various problems. That is, since the halide glass material and the chalcogenide glass material are sudden or abrupt in change in viscosity with respect to temperature change, in a casting temperature range, as compared with the oxide glass material, the halide glass material and the chalcogenide glass material are narrow in a working temperature range. Thus, the conventional methods have a disadvantage that they are difficult in manufacturing of an elongated preform and control of a cladding thickness and core diameter.
As a method of manufacturing a preform, which can solve the above-discussed problems, it has been considered to utilize an extrusion method, as disclosed in Japanese Patent Provisional Publication No. SHO 51-64517 which corresponds to U.S. Pat. No. 4,063,914. In the method disclosed in the publication, however, an enclosing glass material must beforehand be processed into a tubular configuration, and a rod-like or bar-like element of a core glass material must concentrically be arranged within the enclosing glass material. It is technically difficult to form a bore in the glass material. Particularly, since the halide glass material and the chalcogenide glass material are large in thermal expansion coefficient, there are such a disadvantage or problem that the glass material is broken during processing. Furthermore, it is difficult for the method that the cladding glass material and the core glass material are completely in close contact with each other.